Method for designing a template that removably fits to an objects surface

ABSTRACT

The invention relates to a method for designing a template that removably fits to an object&#39;s surface, comprising the steps of: obtaining in a computer aided design system a digitized three-dimensional image of the object determining an approach direction for the template to be placed on the object manipulating the image for removal of undercuts related to the approach direction resulting in a modified image of the object defining the template depending on the shape of the modified image, in which the image is processed for visualization in a viewing direction that identifies with the approach direction, and that the so processed image is used as the modified image depending on which the template is designed.

The invention relates to a method for designing a template thatremovably fits to an object's surface, comprising the steps of:

-   -   obtaining in a computer aided design system a digitized image of        the object    -   determining an approach direction for the template to be placed        on the object    -   manipulating the image for removal of undercuts related to the        approach direction resulting in a modified image of the object    -   defining the template depending on the shape of the modified        image.

Such a method is used for designing parts which have to fit closely tothe surface of an object. The method is known from practice.

The design of the part which is to fit to the object's surface shouldbasically follow the outer contour of the object onto which the part isto be fitted. The problem that associates however to this basicrequirement is that the outer shape of the object's surface is not atall times suited to allow the placement and the subsequent removal ofthe concerning part. In order to be able to place such part on a givenobject and remove it therefrom afterwards, it is sometimes necessary toavoid a precise matching of the template from which the part will bemanufactured, depending on the approach direction.

The problem is illustrated with reference to FIG. 1, showingrespectively on the left a template A, which allows for placing on andsubsequent removal from a surface of an object B. In the middle and inthe right of the FIG. 1, two examples are shown in which a template A,that perfectly matches the outer curvature of the object B, is notcapable to be placed and subsequently removed from the object B.

FIG. 2 shows the same problem illustrated with reference to a varyingapproach direction d of the template A with respect to the object B.Depending on the approach direction that has been elected it is requiredto avoid a precise matching of the outer curvature of the object B inthe template A, in order to be able to place on, and subsequently removetemplate A from object B.

FIG. 3 shows in black areas the parts that should not form part of thetemplate A, in order to allow the placement and subsequent removal ofthe template A from the object B. Said black parts are known in the artas undercuts.

In determining the desired shape of the template A, the prior art takescare of removal of said undercuts in a process such as sweeping orextrusion of the surface of object B in the approach direction. Thecalculation that is required for this process is difficult andcomputationally expensive. The invention is aimed at simplifying thismethod and to make it computationally less expensive.

To this end the method of the invention for designing a template thatremovably fits to an object's surface is characterized in that the imageof the object is processed for visualization in a viewing direction thatcoincides with the approach direction, and that the so processed imageis used as the modified image depending on which the template isdesigned.

The image of the object may have the form of a multidimensional data sete.g. two-dimensional or three-dimensional data set. Such amultidimensional data set assigns data values to respective positions inthe multi (e.g. 2 or 3) dimensional geometric space. Themultidimensional data set in particular represents the spatial shape ofthe object.

The invention is the based on the recognition that between the approachdirection in undercut removal directly corresponds to the visual subjectmatter that comes visible when looking at three-dimensional structuresin a predetermined viewing direction. The parts that are invisible inthe three-dimensional visualization are exactly the undercuts that mustbe removed from the template when it is to fit the visualized object.

An efficient and computationally relatively easy way of removing theundercuts is embodied in a method, which is characterized in that theimage is processed for visualization by determining for each ray in aset of imaginary rays parallel to the viewing direction, the distancefrom a common line of departure of said rays to the surface of the imageof the object, and that the distances for each of said rays arecollected for building the modified image of the object. A modifiedimage of the object which is built this way is devoid of the undercutsthat have to be avoided, and which by their very nature cannot be seenwhen looked at the object in the viewing direction. The templatemanufactured such that it matches the outer surface of the modifiedimage of the object, is capable of being placed on, and subsequentlyremoved from the original object. The set of distances collected in theabove-described manner pertaining to the image of the object, isreferred to in the art as the so-called Z-buffer or Depth Buffer.

The method as just described is particularly useful when the template isa drill guide to be placed on a patient's mandible or maxilla as asurgeon's tool for determining parameters of holes for dental implants.

Beneficially the surface of the mandible or maxilla to which the drillguide is to be placed, is defined in a process of interactivelypositioning graphical representations of dental implants in athree-dimensional image of said mandible or maxilla.

Preferably the approach direction of the drill guide is determined independency of characteristics of the three-dimensional image of saidmandible or maxilla.

The invention also relates to a workstation and to a computer program.The workstation according to the invention is defined in claim 7. Theworkstation according to the invention is able to carry-out the methodof the invention. The computer program according to the invention isdefined in claim 6. When loaded into a computer, the computer programenables the computer to bring about technical effects associated withthe method of the invention. The computer program according to theinvention can be provided on a data carrier such as a CD-ROM, or thecomputer program may be made available via a data network such as theworld-wide web.

The invention will hereafter further be elucidated with reference to thedrawing in which:

FIGS. 1, 2 and 3 illustrate the problem of designing a template of anobject's surface, that allows for placement and subsequent removal, and

FIGS. 4 and 5 illustrate the method according to the invention.

The problem that the invention seeks to solve is elucidated above withreference to FIGS. 1 to 3.

With reference first to FIG. 4 the following discussion of the inventionis offered.

Starting with a digitized three-dimensional image of the object B in acomputer aided design system, the method of the invention processes thisimage that is shown on the left-hand side in FIG. 4, and converts itinto the modified image shown on the right-hand side in FIG. 4. Asmentioned above the basis of the method according to the invention isthe recognition of the similarity between the approach direction inundercut removal and the viewing direction in the visualization ofthree-dimensional structures.

Surface parts which need not be modified for undercut removal given aparticular approach direction are exactly those parts of the surfacewhich are visible after visualization of the three-dimensional object Busing parallel projection along that same approach direction.

The parts, which are invisible in this three- dimensional visualization,are the surface parts that must be modified, and more precisely: whichhave to be extruded in the viewing/approach direction.

For the visualization of three-dimensional structures one canadvantageously make use of an intermediate structure called the Z-bufferor Depth Buffer. This structure is actually an image with the same sizeof the resulting projection image, which holds the distance z along theprojection rays from viewing position to the point where each ray hitsthe surface of the visualized object B. As such, the Z-buffer can beinterpreted itself as a surface in three-dimensions; a two-dimensionalimage with x, y co-ordinates and a pixel value z as a third dimension.The volume z (see right-hand side of FIG. 4) enclosed by the Z-buffersurface and some arbitrary (large enough) value of z can be regarded asan object with the same shape as the above mentioned object B as shownon the left-hand side of FIG. 4.

The Z-buffer is, as mentioned before, automatically generated when avisualization of the object B is calculated. This is the case regardlessof whether the shape of object B is represented as a discrete binaryvoxel volume or by means of a geometric surface description. TheZ-buffer can be described geometrically like a triangulation mesh forinstance. The desired shape of template A can be determined from thereby taking the negative of the shape of the volume Z, which is enclosedby the Z-buffer surface. See FIG. 5, right-hand side.

The beauty of the invention is that it transports visualization methodsthat have evolved to the point that they are quite fast, to the subjectfield of designing templates that have to fit to (complicated) surfacesof objects. Sub-second reconstruction times are common, meaning thatmodification of the visualization direction, including the generation ofa matching Z-buffer, can be done interactively with visual feedback ofthe three-dimensional image of the object B.

The three-dimensional image could be combined with additional graphicsrepresenting relevant information to help the user find the optimalapproach direction. In some situations the required approach directionmay already be known, or may be automatically derived from otherinformation, in which case the interactive determination of approachwith visual feedback can be skipped.

The proposed procedure becomes as follows:

-   Interactive visualization of object B and determination of optimal    approach direction d-   Calculate geometric description (e.g. triangulation) of the Z-buffer    surface-   Determination of the shape of fitting template A as a negative of    the shape of the volume z

With this proposed procedure there is no need anymore to manipulate theshape of a geometrically described surface, which is difficult and acostly part of procedures of the prior art. By means ofthree-dimensional drawing tools and visual feedback of thethree-dimensional image of object B, it is also possible tointeractively indicate the part of the surface of object B which needsto be in contact with template A. This enables another importantoptimization, as then only the relevant part of the Z-buffer surfaceneeds to be triangulated or otherwise described.

The method of the invention can effectively be used in the design of adrill guide, which is to be placed on a patient's gums, teeth, mandibleor maxilla as a surgeon's tool for determining parameters of holes fordental implants. The proposed steps in such an application can than bethe following:

-   Determine position, orientation, length and diameter of holes, which    need to be drilled in gums, teeth, mandible or maxilla of the    patient for placement of dental implants.-   This is done pre-operatively, using three-dimensional visualizations    of the bone in combination with two-dimensional cross-sections    showing structure, density, etc. of the bone and surrounding soft    tissue. These visualizations are for instance derived from a    previously acquired three-dimensional CT scan of the relevant    patient volume. Graphical representations of dental implants can be    positioned and manipulated interactively, all the time visualized as    three-dimensional objects and two-dimensional cross-sections    together with the patient information.-   Indicate which part of the gums, teeth, or bone surface will serve    as a contact area for the drill guide.-   Determine the approach direction visually by manipulating the    three-dimensional view of the patient's anatomy.-   Triangulate the relevant part of the Z-buffer surface.-   Use this surface patch as a basis for automatic design of the    complete drill guide with cylindrical holes at the proper locations,    with proper diameter, etc. which enable the surgeon to drill exactly    according the planning.-   Export the surface description of the drill guide to a manufacturer,    who produces the drill guide and sends it back.-   Place the drill guide in contact with the patients' gums, teeth or    bone during operation, and drill at the planned position, depth,    diameter, etc. by placing the drill in the holes provided by the    drill guide.

As will be clear from the above description the invention is applicableto many areas of technology in which an object must be designed to matchand yet be able to be placed against another object and subsequently beremoved. The invention is therefore considered not to be limited to theabove elucidation but be restricted to the appended claims only whereinthe above elucidation only serves to remove any unclarities.

1. Method for designing a template that removably fits to an object'ssurface, comprising the steps of: obtaining in a computer aided designsystem a digitized image of the object determining an approach directionfor the template to be placed on the object manipulating the image forremoval of undercuts related to the approach direction resulting in amodified image of the object defining the template depending on theshape of the modified image, characterized in that the image isprocessed for visualization in a viewing direction that coincides withthe approach direction, and that the so processed image is used as themodified image depending on which the template is designed.
 2. Methodaccording to claim 1, characterized in that the image is processed forvisualization by determining for each ray in a set of imaginary raysparallel to the viewing direction, the distance from a common line ofdeparture of said rays to the surface of the image of the object, andthat the distances for each of said rays are collected for building themodified image of the object.
 3. Method according to claim 1,characterized in that the template is a drill guide to be placed on apatient's gums, teeth, mandible or maxilla as a surgeon's tool fordetermining parameters of holes for dental implants.
 4. Method accordingto claim 3, characterized in that the surface of the gums, teeth,mandible or maxilla to which the drill guide is to be placed, is definedin a process of interactively positioning graphical representations ofdental implants in a three-dimensional image of said gums, teeth,mandible or maxilla.
 5. Method according to claim 3, characterized inthat the approach direction of the drill guide is determined independency of characteristics of the three-dimensional image of saidgums, teeth, mandible or maxilla.
 6. A computer program comprisinginstructions for obtaining a digitized image of the object determiningan approach direction for the template to be placed on the objectmanipulating the image for removal of undercuts related to the approachdirection resulting in a modified image of the object defining thetemplate depending on the shape of the modified image, characterized inthat the image is processed for visualization in a viewing directionthat identifies with the approach direction, and that the so processedimage is used as the modified image depending on which the template isdesigned.
 7. A workstation arranged to obtaining a digitized image ofthe object determining an approach direction for the template to beplaced on the object manipulating the image for removal of undercutsrelated to the approach direction resulting in a modified image of theobject defining the template depending on the shape of the modifiedimage, characterized in that the image is processed for visualization ina viewing direction that identifies with the approach direction, andthat the so processed image is used as the modified image depending onwhich the template is designed.